Influence of cell-cell contact between L. thermotolerans and S. cerevisiae on yeast interactions and the exo-metabolome.

Sequential fermentation of grape must inoculated with L. thermotolerans and then S. cerevisiae 24 h later (typical wine-making practice) was conducted with or without cell-cell contact between the two yeast species. We monitored cell viability of the two species throughout fermentation by flow cytometry. The cell viability of S. cerevisiae decreased under both conditions, but the decrease was greater if there was cell-cell contact. An investigation of the nature of the interactions showed competition between the two species for nitrogen compounds, oxygen, and must sterols. Volatile-compound analysis showed differences between sequential and pure fermentation and that cell-cell contact modifies yeast metabolism, as the volatile-compound profile was significantly different from that of sequential fermentation without cell-cell contact. We further confirmed that cell-cell contact modifies yeast metabolism by analyzing the exo-metabolome of all fermentations by FT-ICR-MS analysis. These analyses show specific metabolite production and quantitative metabolite changes associated with each fermentation condition. This study shows that cell-cell contact not only affects cell viability, as already reported, but markedly affects yeast metabolism.